1. Field of the Invention
The present invention relates to a run flat tire which can run some distance also in a state in which it punctures.
2. Description of the Related Art
A profile of a tire (a shape of a tread surface which is obtained on the assumption that a groove is not provided) influences the basic performances of a handling stability, a ride comfort and the like. It is necessary to determine a proper profile corresponding to the concept of the tire. Japanese Laid-Open Patent Publication No. 2002-301916 has disclosed a method of determining a tread profile using an involute function. In the profile determined by this method, a radius of curvature of the tread surface is gradually decreased from an equator of the tire toward a tread end. This profile is referred to as a CTT profile. By employing the CTT profile, it is possible to enhance various performances of the tire.
In recent years, a run flat tire comprising a load support layer on an inside of a sidewall has been developed and spread. The run flat tire is referred to as a side reinforcing type run flat tire. When the side reinforcing type run flat tire punctures so that an internal pressure is reduced, a weight of a vehicle is supported by the load support layer. The run flat tire can run some distance also in the case in which it punctures. In a vehicle having the run flat tire attached thereto, it is not necessary to always prepare a spare tire. By employing the run flat tire, it is possible to avoid the exchange of the tire in an inconvenient place. A side reinforcing type run flat tire comprising the CTT profile has been disclosed in U.S. Pat. No. 6,672,351 (Japanese Laid-Open Patent Publication No. 2001-80320).
In a general tire, a tread and a sidewall are flexed during running. In case of the side reinforcing type run flat tire, however, the flexure of the sidewall is suppressed by the influence of the load support layer. In the run flat tire, a tread is mainly flexed. The amount of flexure of the tread of the side reinforcing type run flat tire is larger than that of a general tire. The excessively great flexure of the tread inhibits a handling stability and an abrasion resistance.
In a general tire, a dimension is gradually increased by running. The dimension is increased over both a tread and a sidewall. In other words, the increase is generated in both an axial direction and a radial direction. In case of the side reinforcing type run flat tire, however, the increase in the axial direction is suppressed by the influence of the load support layer. In the run flat tire, the increase in only the radial direction is mainly generated. In the run flat tire, the profile of the tread is permanently distorted by the influence of the deviated increase. As described above, the profile of the tread greatly influences various performances of the tire. In the side reinforcing type run flat tire, various performances in the early stage of use are maintained with difficulty. When the profile is distorted in a tire employing the CTT profile, particularly, the intended performance cannot be exhibited.
It is an object of the present invention to provide a side reinforcing type run flat tire in which an excessively great flexure of a tread during running and a permanent distortion of a profile of the tread are suppressed.